Preparing metal surfaces to resist staining or corrosion



Patented Dec. 22, 1942 PREPARING METAL SURFACES TO RESIST .STAINING ORCORROSION Arthur E. Stevenson, Chicago, 111., assignor to ContinentalCan Company, Inc., New York, N. Y,, a corporation of New York NoDrawing. Application May 9, 1939, Serial No. 272,713

1 Claim.

The invention relates generally to metal treatment and primarily seeksto provide a novel method of treating metallic surfaces, preferably thetinned surfaces of containers or plate to be formed into containers,'soas to form a protective film thereon which will prevent sulphur staininginternally of said containers caused by reaction of the tin with sulphurproducts formed by the decomposition of sulphur containing proteinsduring the sterilizing or processing of the products packaged in suchcontainers.

In the packaging of foodstuffs, of which vegetables, meats, meatproducts and fish constitute good examples, much difliculty isexperienced due to the discoloration or staining of the containers. Thisdiscoloration occurs, over the entire interior of the can but is likelyto be particularly troublesome in the headspace when the products areslack filled in the customary liquor or brine and subjected to prolongedprocessing. This staining is caused by the production of tin and ironsulphides, chiefly tin sulphide, which form a film on the interior ofthe can.

The invention therefore seeks to provide a novel method of chemicallytreating the tin plate containers, or the tin plate from which thecontainers are to be made, whereby a protective film will be formed onthe tinned surfaces Which is effective to definitely prevent theso-called internal sulphur staining or discoloration.

A broad purpose of the invention is to provide a protective film on tinplate or containers formed of tin plate by means of a simplechemicaltreatment thereof with an oxidizing agent in an alkaline media,vand to provide a novel chemical treating material or solution of thatcharacter.

The tinned surface discoloration or staining herein referred to appearsto result chiefly from the formation of tin sulphide by the sulphurcompounds in the foodstufis, and the references herein made to suchdiscoloration or staining are not to be confused with any actual etchingor corrosive attack upon the tinplate sheet itself. It has been foundthat by practicing the method herein disclosed, that is by treating thetin plate or containers with the treating solution herein described, aprotective film will be formed thereon which will prevent the reactionof the tin with sulphur products formed by the decomposition of sulphurcontaining proteins during the sterilizing or processing'of the packagedproducts.

Examples of various alkalies which may be included as alkaline media aretri-sodium phosphate or other alkaline phosphates or any of the causticalkalies, and as the oxidizing agent, a chromate or some modification ofa chromate such as a (ii-chromate or per-chromate, or salts of chromiumin an alkaline solution which also contains some oxidizing material ofsufficient oxidizing power to convert the chromium salts to chromates,may be employed.

In treating the tin plate or the containers formed therefrom accordingto this method, the plates or containers are immersed in the solutioncontaining the alkali and chromate, or they may be sprayed with thesolution, or, in the case of finished containers, they may be filledwith the solution and then emptied. In each case the plates orcontainers would have to be thoroughly washed in order to remove theadhering solution. The treatment should be conducted in a hot solution.It is preferred that the solution' be maintained at a temperature ofapproximately 212 F., but temperatures well below 212 F.

as well as above 212 F. will give satisfactory results although the timeof treatment will necessarily be longer when lower temperatures areemployed.

When treating the containers or tin plate with a solution containingtri-sodium phosphate and sodium chromate, the prevention of staining maybe accomplished in 15 seconds or less at a temperature of 212 F. Fairlygood results have been obtained with,5 seconds treatment and even insome cases with 2 seconds. Different products vary in their stainingeffect on the container, this depending on the character of the productand the time and temperature of the sterilization process. Thepreferable time for covering all conditions would be 2 minutes. Theredoes not seem to be any length of time beyond which the treatment isineffective.

The film produced on the tin plate or conficient results varies widelydepending on the concentration of phosphates in the solution. If we have5 grams -of crystalline tri-sodium phosphate '(Na3PO4JZI-I2O, thiscontains approximately 57 percent of water and is the form in whichtri-sodium phosphate is ordinarily sold) per liter of solution, itrequires 0.5 gram of crystalline sodium chromate (NazCRO-rAHzO,containing approximately 31 percent of water) necessary is less and maydrop somewhat lower than 2.5 percent figured on the crystalline. basisor 3.5 percent figured on the anhydrous basis.

So far as the use of chromate compound together with phosphates areconcerned, a solution 4 containing a minimum of .034 percent of chromatecompound and .215 percent of the phosphate compound (both figured on-theanhydrous basis) with a preferable concentration in the solution used of.86 percent of the phosphate compound and .084 percent of the chromatecompound (both figures on the anhydrous basis) may be stressedasimportant, but it is .considered that the invention is not limited inscope to any particular maximum. To accomplish the desired purpose theproportion of phosphate to chromate need not be fixed but for eachpercentage of phosphate in solution there is a range of percentages ofchromate in solution over which the treatment is efficient. For instance with a solutioncontaining 2 percent of Na3PO4.12Hz0 a percentageof N 8.2C1O4.4H2O from .10 to .30 gives satisfactory results and with asolution containing 8.0 percent of NaaP04.12H:O-, a percentage ofNa=CrO4.4HzO from .20 to 1.0gives satisfactory results. With increasingpercentage of phosphate in the solution, there is an increase in thepercentage range of chromate which will give satisfactory results.

It has been found also that caustic alkalies together with chromates areefiicient infproducing the desired stain or discoloration preventingfilm on tinplate or tin cans. A solution of this nature should include aminimum of .10 percent of caustic a all and .06 percent o'f chromatecompound (calc ated on the anhydrous basis) or a proportion ofapproximately 37.5 percent of chromate comd to hydroxide. The proportionof the chromate compound to the caustic alkali may .be varied, but thelowest proportion of chromate found desirable would be such that itwould make up 20 percent of the total chemicals (chromates and alkalies)in the solution. The preferred combination would be .2 percent causticalkali and .10 percent chromate compound in solution. The invention isconsidered to be not limited in scope to the use of any particularmaximum of caustic alkali or chromate.

In the foregoing description relating to phosphates and causticalkalies, reference is made to chemicals containing water ofcrystallization. Both chromates and phosphates may be purchased withdiffering amounts of water of crystallization. and caustic alkalies withdiflering percentages of moisture. Naturally the percentage of waterpresent in the chemicals will influence the amount of each chemical usedand the proportion of one to the other.

The phosphates may be used in the form of either potassium tri-phosphateor sodium triphosphate. The latter is very much cheaper than the former.Chromates could be used as ammonium chromate, potassium chromate orsodium chromate, or as .di-chromates or per-chromates of any of thesealkalies.

. Extensive experiments have shown that when tin'plate or the containersformed therefrom are Patent is:

basis and substantially 0.084 percent of sodium chromate on theanhydrous basis for a time not essentially exceedingntwo minutes, andthen thoroughly rinsing the tin plate before contact with the saidproducts.

, ARTHUR E. STEVENSON.

